Field of the Invention
The present invention relates generally to superconductors, and more particularly to a magnetic field source formed from a flexible closed superconducting ribbon.
Description of the Prior Art
Superconductivity was first observed in 1911 by a Dutch scientist by the name of Heike Kamerlingh-Onnes who observed the disappearance of all electrical resistance from a thin capillary of mercury metal located in a bath of liquid helium. More recently, superconductivity has become an active area of technology with the discovery of high temperature superconductive materials in 1986 when K. Alex Mueller and J. George Bednorz synthesized a complicated compound and found superconductivity to occur therein in the region of 35.degree. Kelvin. This was followed shortly thereafter in 1987 when Maw-Kuen Wu and Paul Chu observed a transition to the superconducting state using only liquid nitrogen as a coolant.
Two effects are characteristic of the superconductivity phenomenon, i.e. it is associated with perfect conductivity and perfect diamagnetism. Perfect conductivity means that the material exhibits zero electrical resistance while perfect diamagnetism means that a device in a superconductive state excludes all magnetic fields from its interior. The latter effect results from internally generated currents that produce opposing magnetic fields. However, it is also well known that superconductors can only carry so much current or withstand so much external magnetic field before losing their unique characteristics of perfect conductivity and perfect diamagnetism. These properties of superconductors are called critical currents and critical magnetic fields. Thus when a superconductor carries a current which is equal to a value called the critical current I.sub.c, its superconductivity abruptly vanishes. Alternatively, when a strong external field, termed the critical field H.sub.c, is applied, superconductivity vanishes once again. However, both the critical current and critical field are functions of temperature and more particularly, a critical temperature T.sub.c.
It is generally known that flux can be normally trapped and confined in superconducting rings in one of two ways. The first is when the ring is placed within a magnetic field H with its axis aligned with the field while the temperature is above the transition or critical temperature T.sub.c of the material from which the ring is fabricated, and after which the temperature is lowered below T.sub.c and the field H removed. This leaves the ring with trapped flux and a persistent current flowing in the ring. The second way is to place a ring that is below its transition temperature T.sub.c into an axial field H that is greater than the critical field H.sub.c and then to reduce the field H to zero. This is accompanied by the ring becoming superconductive as the field drops below H.sub.c leaving a flux .theta.=H.sub.c A trapped in the ring where A is the interior cross sectional area of the ring.
In the above cross referenced related application U.S. Ser. No. 07/673,422, there is disclosed the concept of varying the inner cross sectional area formed by a superconductive loop including a section of relatively low critical field material. Reduction of the internal area increases the magnetic field at the surface of the loop. When the field at this section reaches H.sub.c, it becomes normal and magnetic flux leaks from within the loop through the low critical field section. This enables one to detect and measure the H.sub.c of a test piece which forms the section of low critical field.